As one of photoelectric conversion elements, there is a solar cell and, as a material thereof, there is used a III-V group compound semiconductor, including monocrystalline, polycrystalline or noncrystalline silicon (Si), germanium (Ge), and GaAs, a II-VI group compound semiconductor, such as CdS, or the like. As the device structure of the solar cell, there are known a pn junction, pin junction, a tandem structure, and the like.
In recent years, efforts are being made to develop photoelectric conversion elements making use of Ge, with the aim of realizing the high efficiency of solar cells by effectively utilizing the optical spectrum of sunlight. For example, there have been proposed a tandem-type solar cell in which Ge is used in the bottommost layer thereof (GaInP/GaAs/Ge-based solar cell) (R. R. King et. al., “Metamorphic GaInP/GaInAs/Ge Solar Cells.” Proc. 28th IEEE Photovoltaic Specialists Conf. (IEEE. New York, 2000), p. 982) and a tandem-structured photoelectric conversion element laminated with a silicon crystal cell for photoelectrically converting light having a wavelength of approximately 1.1 μm or less and a germanium-based crystal cell for photoelectrically converting sunlight having a waveband of 1.1 to 1.6 μm (for example, Japanese Patent Laid-Open No. 6-291341).
FIG. 1 is a cross-sectional schematic view used to explain the structure of a conventional tandem-structured photoelectric conversion element laminated with a silicon crystal cell and a germanium-based crystal cell. This element has a tandem structure in which a germanium-based crystal cell 2 for absorbing and photoelectrically converting light (hv1) having a band of wavelengths greater than 1.1 μm and a silicon crystal cell 1 for absorbing and photoelectrically converting light (hv2) having a band of wavelengths equal to or less than 1.1 μm are laminated. These cells are pn junction type cells in which layers (1A and 1B, and 2A and 2B) of mutually opposite conductivity types (“p” type and “n” type) are respectively laminated. A pn junction is also formed at a boundary face between the silicon crystal cell 1 and the germanium-based crystal cell 2.
In the upper portion of the silicon crystal cell 1 which is a top cell, there is provided a transparent electrode 3 so as to be held between a glass plate 4 and the silicon crystal cell 1. In the lower portion of the germanium-based crystal cell 2 which is a bottom cell, there is provided an electrode 5. Light entering from the transparent electrode 3 side is photoelectrically converted within the top cell and the bottom cell, and carriers thus generated are taken out through the electrodes (3, 5) as an electric current.
When forming the silicon crystal cell and the germanium-based crystal cell into a tandem structure, the photoelectric conversion element is constructed using the germanium-based crystal cell as a bottom cell. This is in order to allow relatively high-energy light (hv2) to be photoelectrically converted in an efficient manner within the silicon crystal cell and to allow relatively low-energy light (hv1), which is not photoelectrically converted by the silicon crystal cell, to be photoelectrically converted by the germanium-based crystal cell.
For this reason, a silicon layer is generally grown epitaxially on a single-crystal germanium-based substrate when fabricating a tandem-type photoelectric conversion element having such a structure as shown in FIG. 1.
However, a single-crystal germanium-based substrate has such problems that it is costly and scarce and that it is not available as a large-diameter substrate.
As means for solving these problems, it is possible to fabricate a tandem-type photoelectric conversion element having an Si/Ge-based structure using a substrate in which a germanium-based (Ge-based) crystal has been epitaxially grown on a silicon (Si) single-crystal substrate. In that case, there arise the following problems, however.
First, it is extremely difficult to further epitaxially grow Si on germanium (Ge) epitaxially grown on a silicon (Si) substrate. This is because the melting point of Ge (approximately 910° C.) is significantly lower than a temperature generally required for the epitaxial growth of Si.
Gases used for the epitaxial growth of Si include SiH4, SiH2Cl2, SiHCl3 and SiCl4. The optimum decomposition temperatures of these gases are defined respectively as SiH4: 950 to 1000° C., SiH2Cl2: 1050 to 1150° C., SiHCl3: 1100 to 1200° C., and SiCl4: 1150 to 1250° C. (according to M. L. Hammond, “Silicon epitaxy”, Solid State Technol., Nov., pp. 68-75 (1978)). Accordingly, if Si is epitaxially grown at these temperatures, the underlying Ge epitaxial layer melts down. Consequently, it is unavoidable to perform the epitaxial growth of Si at a temperature lower than the optimum decomposition temperature and, therefore, it is difficult to obtain an Si layer of high crystal quality.
Alternatively, as shown in FIGS. 2(A) to 2(D), a silicon layer 1B whose conductivity type is opposite to that of the silicon single-crystal substrate 1A is previously formed thereon and layers of germanium-based crystal (2A, 2B) having mutually different conductivity types are laminated on this silicon layer (FIG. 2(A)). A supporting substrate 6 is bonded to a surface of the germanium-based crystal layer 2 (FIG. 2(B)) to make the crystal layer capable of being handled. The rear surface of the silicon single-crystal substrate 1A is polished (FIG. 2(C)) to a predetermined film thickness. Consequently, there is obtained a substrate for photoelectric elements suited for construction in which the silicon crystal cell serves as the top cell and the germanium-based crystal cell as the bottom cell (FIG. 2(D)). However, steps of polishing the rear surface of the silicon single-crystal substrate 1A and then cleaning the substrate after polishing are essential in such a process as described above. This essentiality not only makes a process for manufacturing a substrate for photoelectric conversion elements cumbersome and complicated but also raises the cost of manufacture. In particular, this essentiality makes it difficult to meet the needs for “cost reductions” required of a solar cell.
The present invention has been accomplished in view of the above-described problems. It is therefore an object of the present invention to provide a method for manufacturing a substrate whereby it is possible to provide a photoelectric conversion element having an Si/Ge-based structure at low costs.